12 Assertions and Errors

This section describes two macros that test assertions, i.e.,
conditions which must be true if the program is operating correctly.
Assertions never add to the behavior of a Lisp program; they simply
make “sanity checks” to make sure everything is as it should be.

If the optimization property speed has been set to 3, and
safety is less than 3, then the byte-compiler will optimize
away the following assertions. Because assertions might be optimized
away, it is a bad idea for them to include side-effects.

— Macro: cl-assert test-form [show-args string args...]

This form verifies that test-form is true (i.e., evaluates to
a non-nil value). If so, it returns nil. If the test
is not satisfied, cl-assert signals an error.

A default error message will be supplied which includes test-form.
You can specify a different error message by including a string
argument plus optional extra arguments. Those arguments are simply
passed to error to signal the error.

If the optional second argument show-args is t instead
of nil, then the error message (with or without string)
will also include all non-constant arguments of the top-level
form. For example:

(cl-assert (> x 10) t "x is too small: %d")

This usage of show-args is an extension to Common Lisp. In
true Common Lisp, the second argument gives a list of places
which can be setf'd by the user before continuing from the
error. Since Emacs Lisp does not support continuable errors, it
makes no sense to specify places.

— Macro: cl-check-type form type [string]

This form verifies that form evaluates to a value of type
type. If so, it returns nil. If not, cl-check-type
signals a wrong-type-argument error. The default error message
lists the erroneous value along with type and form
themselves. If string is specified, it is included in the
error message in place of type. For example:

(cl-check-type x (integer 1 *) "a positive integer")

See Type Predicates, for a description of the type specifiers
that may be used for type.

Note that in Common Lisp, the first argument to check-type
must be a place suitable for use by setf, because
check-type signals a continuable error that allows the
user to modify place.